This invention relates to a method of synthesizing 4-substituted phthalic anhydrides. In particular, it relates to a method of synthesizing 4-chlorophthalic anhydride (4-CPAN) from chloromaleic or bromomaleic anhydride, chloroprene, and an oxidizing agent such as air, oxygen, chlorine or bromine. Similarly, 4-fluorophthalic anhydride (4-FPAN) can be prepared from chloro or bromomaleic anhydride, hydrogen fluoride, chloroprene, and an oxidizing agent.
4-CPAN and 4-FPAN are valuable intermediates for the production of various specialty chemical products. While it is possible to prepare 4-CPAN by directly chlorinating phthalic anhydride, the principal products tend to contain di- and tri-chlorinated phthalic anhydrides as well as 3-chlorophthalic anhydride. While other methods exist for the formation of 4-CPAN by direct chlorination, these methods do not result in easily purified 4-CPAN. 4-FPAN can be prepared by the halogen exchange reaction of 4-CPAN with potassium fluoride.
In another approach (see U.S. Pat. Nos. 5,003,088 and 5,049,682) for preparing a 4-halophthalic anhydride, chloroprene is reacted with maleic anhydride to produce a substituted cyclohexene which is oxidized with a halogen such as chlorine or bromine. While this process successfully produces the-desired 4-halophthalic anhydride, it requires the use of a halogen source such as chlorine or bromine in the aromatization reaction and produces hydrochloric or hydrobromic acid as a byproduct, as well as other byproducts, and it is difficult to separate the 4-halophthalic anhydride from the byproducts.